Electrocatalysis and Forced Convection Measurements: A Guide to Complete Solutions

Electrocatalysis can be defined as the catalysis of an electrode reaction. The electrocatalytic leads to an increase of the standard rate constant of the electrode reaction, resulting in a higher current density, or to decrease in over potential when other rate limiting steps are involved. The examination of an electrocatalytic process requires knowledge regarding the influence of electrode properties on the mechanism and kinetics of the electrode reaction. Forced convection techniques offer the benefit of decreasing the contributions from mass-transport and providing a direct access to kinetic and mechanistic information. Metrohm Autolab provides a comprehensive range of products for these demanding experiments.

Oxygen Reduction Reaction (ORR) – an Essential Electrochemical Reaction

The oxygen reduction reaction (ORR) is the most crucial reaction in life processes such as biological respiration, in energy converting systems such as fuel cells and in corrosion. ORR in aqueous solutions happens mainly by two pathways: the direct four-electron reduction pathway from O₂ to H₂O, and the two-electron reduction pathway from O₂ to hydrogen peroxide (H₂O₂). In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O₂ to superoxide (O₂^-) can also happen.

ORR is known to suffer from very sluggish kinetics and large over potentials are needed to accomplish a decent turnover rate on the electrode. For energy conversion processes, these kinetics need to be enhanced to attain a practical usable level in fuel cells.

Existing research focuses on platinum, the ideal catalyst for ORR, or combinations of this metal with other materials, as well as on the application of non-noble metal catalysts.

Forced convection has been the technique of choice for investigating this and other reactions. Rotating disk and rotating ring-disk measurement offer valuable insights in the mechanism and kinetics involved in the reaction.

The experimental characterization of these electrocatalysts requires the cleanest possible conditions as impurities may cause inaccurate data and conclusions.

In this article, information regarding how one can perform these experiments using the dedicated Metrohm Autolab products will be discussed.

ECAT-Compact – Basic Package for Electrocatalysis Experiments

The compact PGSTAT204, fitted with the dual mode BA bipotentiostat module offers a convenient solution for electrochemical measurements using forced convection. The rotating ring-disk electrode (RRDE) can be remotely regulated by the instrument and the BA module offers the possibility to perform both collection-detection experiments and shielding experiments.

• Small but powerful - Despite its small footprint, the PGSTAT204 provides all the functionality needed for high quality and accurate electrochemical measurements.
• Built-in analog integrator - For charge and cyclic voltammetry current integration measurements.
• Dual mode bipotentiostat - The BA module can be used to regulate the second working electrode in scanning bipotentiostat mode or bipotentiostat mode.
• Accurate motor controller - High precision control of the rotation rate, from 100 to 10 000 RPM.
• Monitor cable - Provides connections for analog signals and control for the Autolab motor controller.
• Electrochemical cell - Special electrochemical cell for electrocatalysis measurements.
• Waterbath control - Software interface to external waterbath for temperature regulation.
• Generation collection or shielding - The rotating ring-disk electrode can be used for both generation collection or shielding experiments.
• Powerful software - Advanced electrochemical NOVA 2 control software with integrated Levich and Koutecký-Levich analysis techniques.

ECAT-Complete – Complete Package for Electrocatalysis Experiments

The ECAT-complete packages offer extra functionality with regards to the compact version. The extra SCAN250 module offers true linear scan cyclic voltammetry for both disk and ring. The FRA32M module, also included, incorporates electrochemical impedance spectroscopy to the list of electrochemical methods supported by the instrument. The pX1000 module provides the possibility to link a pH sensor and temperature sensor so as to monitor these crucial experimental parameters at all times.

• Modular system - At any time, the Modular Autolab PGSTAT302N can be expanded with extra functionality.
• pH and temperature measurement – The pX1000 module offers real time monitoring of temperature and pH.
• True linear scan capable - The SCAN250 module offers unique true analog scan functionality for correct interfacial electrochemistry measurements.
• Waterbath control - Software interface to external waterbath for temperature regulation.
• Electrochemical impedance spectroscopy - The FRA32M module extends the instrument functionality with electrochemical impedance spectroscopy.
• Generation collection or shielding - The rotating ring-disk electrode can be used for both generation collection or shielding experiments.
• Analog input/output - Offers connections for analog signals and control for the Autolab motor controller.
• Electrochemical cell - Dedicated electrochemical cell for electrocatalysis measurements.
• Powerful software - Advanced electrochemical NOVA 2 control software with built-in Levich and Koutecký-Levich analysis methods.

Forced Convection Methods and Experiments

When an electrode immersed in a solution rotates, a convective drag is formed. This in turn forms drag from the bulk of the solution towards the surface of the electrode. Electroactive species are thus transported towards the surface of the electrode by convection and diffusion, with a diffusion layer thickness based on the rotation rate of the electrode. The reaction products and intermediates or surplus reactant are then ejected outwards back to the bulk of the solution. When this happens on a rotating ring-disk electrode, an additional reaction can take place when reactants or products are transported over the ring.

Generation/Collection Experiment

In the most common use of the RRDE, products formed on the disk can be detected on the ring, provided that the ring conditions are modified accordingly. The oxygen reduction reaction (ORR) is a classic example of this experimental arrangement, where the ring is used to detect the formation of H₂O₂. This provides valuable insights in the reaction pathway.

Shielding Experiment

An alternative application of the RRDE involves two consecutive measurements. In the first measurement, only the ring is polarized and the ring current is recorded. In the following measurement, the disk is also polarized at the same potential. Both electrodes then contest against one another, with the disk shielding the reaction on the ring. The oxidation of NaBH₄ provides a good illustration of this use of the RRDE.

RRDE Cell – Measurements Done Right

Electrocatalysis experiments with noble metals and platinum in particular must be carried out in the cleanest possible conditions. With a total volume of 250 mL, the Autolab RRDE cell meets this requirement:

• Easy assembly and disassembly- The cell can be easily assembled and can be fully taken apart for cleaning
• The maximum chemical resistance- All parts in contact with the solution are composed of materials with the ideal chemical resistance. These can be cleaned in the extreme conditions dictated by the experimental needs.
• Stable measurement environment- When assembled, the RRDE cell offers a stable experimental environment ideal for electrochemical measurements under reproducible laminar flow conditions
• Two-way gas purge- The included gas purge tube can be used to saturate the solution with a gas, usually oxygen and to blanket the solution once saturated. For rapid gas saturation, the gas purge produces micro-bubbles using a small glass frit.
• Thermostatic jacket- The glass cell includes a thermostatic jacket for thermal control of the measurement conditions

RRDE – Ultra-Low Noise Rotator and Controller

The Autolab rotating ring-disk (RRDE) rotator employs a double, friction-less, mercury-based electrical contact for both electrodes. This guarantees ultra-low noise measurements even in the most challenging experimental conditions.

• Compatible- The RRDE controller can be used along with any Autolab potentiostat/galvanostat.
• Full control- The RRDE rotator can be regulated manually using the Autolab motor controller or remotely using the NOVA 2 software.
• Ideal fit- The RRDE rotator is built to fit in the matching conical fittings placed in the RRDE cell. This guarantees stable measurements conditions irrespective of rotation rate.
• Accurate control- The rotation rate can be regulated with a precision of a single rotation per minute (RPM) between 100 and 10 000 RPM, with an acceleration and deceleration rate of 4 000 RPM/s.

Ring Disk Electrodes – The Key to All Experiments

Electrocatalysis experiments can be conveniently carried out using the ring-disk electrode. Products and reaction intermediates developed on the disk can be detected on the concentric ring in hydrodynamic conditions.

• Wide rotation rate range- The ring disk electrodes are built to be used in the full rotation rate of the Autolab RRDE rotator, from 100 RPM to 10 000 RPM.
• Accurate construction- The electrodes are assembled using the most precise construction techniques. A 375 µm gap between disk and ring results in a theoretical collection efficiency of 24.9%.
• Different materials- The electrodes are available with gold, glassy carbon or platinum as disk material and platinum as ring material.

Autolab NOVA – Advanced Electrochemistry Software

The NOVA software is used to regulate all Autolab potentiostat/galvanostat and accessories connected to it. It provides a simple and advanced interface for all electrochemical experiments and is provided with a wide library of techniques ideal for a broad range of applications.

Procedures for measurements with the rotating disk or the rotating ring-disk electrode are readily available. These techniques use the remote control option of the motor controller to automatically scale up the rotation rate during the experiment, using a square root distribution.

NOVA gives answers in real-time. The detection of a reaction intermediate can be observed as the measurement continues.

Procedures provided for rotating ring-disk measurements include data handling and data analysis commands that carry out the essential steps to create Levich and Koutecký-Levich plots together with the experimental data recorded with the RRDE or RDE.

These plots, provide information on the number of exchanged electrons or the kinetic constant. This way, a single experiment offers all of the information required to provide insights on the kinetics and the reaction pathway.

Features

Specifications and Features Overview

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.